A conventional dump vehicle has a frame mounted on wheels with a driver's cab, and a dump box that can hold a volume of material that is transported to a location and dumped. A hydraulic lifting system on the dump vehicle pivots the dump box with respect to the frame to an angle where the loaded material can freely slide out of the rear end of the dump box. This type of dump box has a front wall and cab guard, two sidewalls with a tailgate pivotally mounted to a rear upper end portion of the dump box. A latching device is provided near the bottom edge of the tailgate. The latch is used to hold the tailgate closed so that the loaded material will not fall out during transit.
Dump trucks haul various materials into and out of construction projects for building pads, parking lots, road structures, etc. Some of the time, these trucks stockpile the loaded material in piles by releasing the tailgate latch and raising the dump box to an angle where the material freely sides out through the tailgate opening and onto the ground. Other times these trucks are required to spread the loaded material out evenly as the truck is in motion so as to provide an even discharge of material over a desired area. To enable the truck for this material spreading, a driver will usually stop the truck, get out of the cab and manually set a pair of spreader chains on the rear end of the dump box. One end of each of the chains are most often found welded to the back end of the dump box frame, and the other end is attached to the tailgate by hand in a connecting slot that is provided on the tailgate. The truck driver selects a certain length of chain and attaches it to each side of the tailgate. The driver then returns to the cab with the tailgate latch still closed. The dump box is then raised to about a 40 degree angle above the truck frame. The truck is then set in motion, the tailgate latch is released, and the (pre-set) spreader chains stop outward movement of the tailgate so as to cause metered material to exit. The chains are set to cause the dump box to release an exact depth and flow rate of material. Different chain length settings allow the material to be spread out at different thickness as needed. It should also be noted that dump trucks are often used to haul materials in both directions, i.e., two and from a worksite. Indeed, there are times when there is as much or more waste material leaving a job site then that which comes in. This means that drivers are required to set chains when arriving at the site, e.g., to spread refined products, and then to remove the chains when leaving so that the waste material can be dumped from the truck bed. This is an inconvenience, loss of time, and puts the driver outside the safety of the truck and in harms way.
A spreader system sets an opening by essentially dropping the tailgate from an inclined dump box until it is suddenly restrained by the spreader chains. The resulting jolt or impulse force felt by both the tailgate and dump box provides a beneficial effect. It tends to break up packed down material and thus promotes metered flow from the dump box at the moment when the truck begins moving. But each time a dump truck driver is required to make adjustments to the tailgate opening, or when changing from spreading material to dumping material out in a pile, the driver must stop the truck, get out, and manually reset or un-set the spreader chains. Nevertheless, spreader chains are still widely used today because they are believed to work well—an even spread of loaded material can be achieved using chains.
Two other systems for spreading or dumping material have been proposed. They are described in U.S. Pat. No. 6,499,808 to Palmberg and U.S. Pat. No. 6,302,491 to Anderson, respectively Palmberg discloses a dump vehicle with a tailgate that is pivotally mounted adjacent to the rear opening of a bed. The tailgate has a closed position in which a gate blocks the rear opening to prevent discharge from the bed through the rear opening. The vehicle further includes a gate control mechanism connected to the gate. The gate control mechanism selectively raises the gate from a closed position to a first position using a hydraulic cylinder, allowing metered discharge of material, or to a second position that allows the gate to fully pivot so as to allow unmetered discharge of material. Palmberg's device may be operated with chains or with the gate control mechanism. When the gate control mechanism is used, the tailgate can be selectively raised to a desired height by causing the tailgate to travel linearly along channels formed on the bed. Additionally, Palmberg's system appears to operate by a hand-eye system when finding the correct opening for metered flow.
Anderson discloses a control system for controlling the opening and closing of the tailgate of a dump truck including an actuator unit, which is mounted on the truck box or tailgate. The actuator unit controls the degree of opening of the tailgate during an unloading operation and therefore the rate at which the material is discharged from the truck box. A positional feed back unit produces an output signal indicating the movement of the tailgate between open and closed positions. This signal is displayed on a control panel of a control unit in the truck cab. An operator in the truck can control the opening and closing of the tailgate and can monitor the position of the tailgate. The control system is said to effect an accurate metering of material from the truck box.
Anderson's device does not have a separate latch to keep the tailgate closed during transit. Instead, the proposed system for metered flow also closes the tailgate. Thus, in Anderson's system, a hydraulic system controls both the flow metering as well as the latching or unlatching of the tailgate. Also, Anderson's system uses a pair of double acting hydraulic systems to raise and lower the tailgate.
It would be desirable to have a system that retains all of the benefits of a chain system while removing the less desirable qualities of such a system for dumping material; it would also be desirable to have a reliable system for dropping metered flow, without an increased cost in man hours associated with assuring proper dumping and/or spreading of material; it would also be desirable if a system were available that did not require extensive retrofitting to existing trucks or significant changes to design; it would also be desirable if a system were available which did not place a driver in harms way when resetting a gate; and/or it would be desirable if a system were provided which offered a fail-safe mode which allowed the truck to be used even if there is a loss of power in a tailgate opening mechanism.